The invention relates to a drill screw for fastening a structural member to a, in particular, harder supporting member, comprising a head and a shank joining the head and having a self-cutting machine thread which is flattened to have a decreasing depth of thread and decreasing outer diameter in a drilling section joined by a tapered drill bit, with the axial length of the drill section being approximately equal to the thickness of the structural member to be fastened and the axial position and length of a thread principal portion disposed closer to the head and formed with full depth of thread and full outer diameter being dimensioned such that with the drill screw in the fully threaded-in condition the supporting member is threadedly connected to the thread principal portion, with two diametrically opposed chip grooves having an approximately rectangular cross sectional area being furthermore provided in the drill section and the drill bit, and a transverse cutting edge extending between the chip grooves provided at the free end of the drill bit, the chip groove areas disposed to the rear when looking in the direction of screwing-in of the screw furthermore forming cutting edges with the portions of the drill section that were left untouched between the chip grooves and the drill bit, and with the conical basic shape of the drill bit being furthermore preferably modified by flats extending from the transverse cutting edge.
Such a drill screw has become known from the German publication letter No. 13 03 320. Also the German publication letter No. 14 00 853 shows a similar drill screw, however, not with a machined thread (V-thread) but with a sheet metal screw thread.
When using the known drill screws of the type as indicated above cases have arisen in which during the initial phase of performing the drilling or turning action in the harder supporting member the drill bit was damaged and thereby the further drilling or screwing-in was rendered difficult. The near-at-hand remedy of meeting these disadvantageous possibilities by using a material of higher strength for the drill screw necessitates an unreasonably high amount of expenses which is not tolerable with the usual applications of drill screws; drill screws are typical mass production articles which are normally used only once or at best a few times.
It is the object of the invention to provide a drill screw for fastening a structural member on a preferably harder supporting member with which the danger of damaging the drill bit is strongly reduced at low cost.
According to the invention this object is attained with the aid of a drill screw of the type mentioned at the beginning which is characterized in that the bottom in each chip groove is occupied by a flute the outer surface of which forms a transverse area extending between the groove side walls and forming in common with them two inner edges, and in that the transverse cutting edge extends between the inner edges disposed to the rear looking in the direction of screwing-in of the screw.
It has been found that with the drill screw according to the invention the danger of the drill bit getting damaged is very strongly reduced and practically eliminated. According to the present state of knowledge this is to be attributed above all to the fact that due to the flute occupying the chip groove bottom the strength is relatively strongly increased just in the particularly endangered region, especially in the drill bit. The screw is therefore in a position with a higher degree of safety to withstand the functionally conditioned load peaks, especially when stripping the thread pre-cut in the structural member to be fastened; this effect is still enhanced by the fact that the machined thread used with the screw according to the invention has a finer thread lead than the sheet metal screw threads otherwise largely used in connection with drill screws, so that the compulsory feed first occurring when drilling into the supporting member is correspondingly reduced through the thread pre-cut in the fastening member. Accordingly, the advantages of the drill screw according to the invention are particularly significant vis-a-vis drill screws having a wood or sheet metal screw thread (for instance, German publication letter No. 14 00 853). The screw according to the invention unexpectedly still shows the additional advantage that harder and thicker supporting members may be used without specific cutting edges having to be provided at the drill screw within the thread; this advantage, too, according to the present state of knowledge is to be attributed to the altogether strongly increased loading capacity of the drilling portion and the drill bit.
The production of the screw according to the invention is not more difficult than that of known drill screws of the same species; especially, the chip grooves provided with a flute can be produced by cold-forming processes in the same manner as the usual chip grooves having a fully relieved bottom. The advantages of the drill screw according to the invention are obtained solely through constructional measures and not through selection of a material of higher strength and thus a correspondingly more expensive material.
In a further development of the invention a still higher increase of the loading capacity may be obtained in the particularly endangered region of the drill section and the drill bit by enlarging the thread core diameter in the drilling section towards the drill bit. This not only reduces still further the depth of thread towards the drill bit so that the drilling and the initial phase of thread cutting are facilitated as is the stripping of a pre-cut thread but also the reduction of the material cross sectional area which due to the course of the chip grooves increases towards the drill bit is at least in part compensated for; this clearly contributes to the increase in strength.
The invention is described in more detail in the following specification by way of an example as shown in the accompanying drawings.